1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having thin-film transistors used as switching elements.
2. Description of the Related Art
Liquid crystal display devices have a liquid crystal layer sandwiched between two electrodes which apply an electric field to the liquid crystal layer for controlling the transmittance degree of light that passes through the liquid crystal layer.
One system for applying an electric field to the liquid crystal layer is known as a static drive system which constantly supplies a fixed voltage signal to each of the electrodes. If the liquid crystal display device driven by the static drive system is designed to display a large amount of information, however, it requires a huge number of signal lines to be connected to the electrodes.
Heretofore, a liquid crystal display device for displaying a large amount of information is associated with a multiplex drive system which supplies signal voltages on the multiplex time-division principles.
One type of such a multiplex drive system is referred to as an active-matrix drive system which holds an electric charge applied to an electrode until a next frame. The active-matrix drive system allows the liquid crystal display device to display images of high quality. Some liquid crystal display devices that are driven by the active-matrix drive system employ thin-film transistors (TFT) which have excellent charge holding characteristics. Such TFT liquid crystal display devices are used as display devices which are required to display high-contrast images of high quality.
FIG. 1 shows a cross section of a general TFT liquid crystal display device. A polarizer (polarizing film), etc. are omitted illustration in FIG. 1.
The TFT liquid crystal display device shown in FIG. 1 comprises an insulating film 12 of silicon nitride, for example, disposed on a glass substrate 10. Transparent electrodes 11 (also referred to as pixel electrodes) are arranged in a matrix on the insulating film 12, making up matrix segments.
An amorphous silicon film 13 is also disposed on the insulating film 12. A plurality of longitudinal drain electrodes 14 are disposed on the insulating film 12 in overlapping relation to the amorphous silicon film 13, and are connected to drain lines (not shown), which may be referred to as data lines or signal lines.
A source electrode 15 is connected to the transparent electrodes 11 in overlapping relation to the amorphous silicon film 13.
A gate electrode 17 is formed between the glass substrate 10 and the insulating film 12, and connected to a plurality of transverse gate lines (not shown), which may be referred to as scan lines.
The gate electrode 17 is disposed underneath the amorphous silicon film 13 at a gap between the source electrode 14 and the drain electrode 15.
As shown in FIG. 5 of the accompanying drawings, a drain line D, a gain line G, a source line S, and an amorphous silicon film connected to these lines D, S, G jointly make up a thin-film field-effect transistor (FET) which serves as a switching element (switching transistor).
The transparent electrodes 11 are connected to the drain line through the switching elements .
In FIG. 1, the switching elements, the drain line (drain electrode), and the gate line (gate electrode) are covered with and protected by a passivation film 16 of silicon nitride.
In order to orient liquid crystal molecules, an orientation film 18 of an organic material is disposed on the passivation film 16.
A glass substrate 20 supports a transparent common electrode 21 and an orientation film 28 on its lower surface facing towards the glass substrate 10. A liquid crystal layer 3 is sealed between the orientation films 18, 28.
When the switching transistor of each of the matrix segments is turned on or rendered conductive, an electric field is developed between the transparent electrodes 11, 21, causing the liquid crystal layer 3 to produce an electro-optic effect to display an image on the entire TFT liquid crystal display device.
As shown in FIG. 2, a DC voltage which is identical to the central value of a pixel electrode potential is applied to the common electrode 21 at an intermediate tone. A potential (Vcom) of the common electrode 21 with respect to a pixel electrode potential is shown in FIG. 3.
FIG. 4 shows the conventional liquid crystal display device which includes a circuit for applying the DC voltage to the common electrode 21.
As shown in FIG. 4, the circuit includes a voltage offset circuit 31 connected as a voltage divider between a power supply and ground for producing a variable voltage. The voltage offset circuit 31 sets the central potential value (intermediate potential) of the common electrode 21 to the central value of the pixel electrode potential (see FIG. 2) for displaying an intermediate tone.
However, the conventional liquid crystal display device with the voltage offset circuit for setting the central potential value of the common electrode to the central value of the pixel electrode potential is disadvantageous in that a displayed pattern causes a residual image to be left for a long period of time, degrading display characteristics. Such a residual image is explained below.
The potential central value differs with the displayed gradation. Therefore, when a gradation pattern other than the intermediate tone is displayed, a DC component is applied to the liquid crystal cells within the gradation pattern, causing impurity ions in the liquid crystal cells or the orientation films 18, 28 to produce an electric double layer which results in an internal potential. The internal potential varies the effective voltage in the pattern, producing a brightness difference.
Japanese Patent Laid-open No. 149983/1989 discloses an arrangement for attracting impurity ions to one side of a liquid crystal display panel under an internal electric field.
According to the disclosed arrangement, however, if the orientation films have a high ion absorption capability, then impurity ions are absorbed to the orientation films while they are being attracted to one side of the liquid crystal display panel, thereby tending to produce an electric double layer. If the orientation films are prone to fixed polarization, then they are unable to suppress a residual image that is left for a long period of time.